1. Field of the Invention
The present invention relates to a vibration wave driving apparatus and, in particular, to a vibration wave driving apparatus configured of a rotor of which one end comes into contact with a vibrator and which is frictionally driven due to the vibration excited to the vibrator and an output transmitter which engages the other end of the rotor and transmits the output of the rotor externally.
2. Description of the Related Art
Generally, a vibration wave motor that is a vibration wave driving apparatus is applied to a product for driving a camera lens or the like. In addition, an annular type and a rod type thereof exist. Hereinafter, the rod type vibration wave driving apparatus of the prior art is described (for example, Japanese Patent Application Laid-Open No. 2002-291263). FIG. 3 is a cross-sectional view illustrating a configuration of the vibration wave driving apparatus using the rod type vibrator of the prior art.
In FIG. 3, the vibration wave driving apparatus has a first elastic body 301, a second elastic body 302, a piezoelectric element 303 (an electrical-mechanical energy converting element), a shaft 304, a lower nut 305 and an upper nut 311.
The first elastic body 301, the second elastic body 302 and the laminated piezoelectric element 303 are tightened by the shaft 304 and the lower nut 305 to apply a predetermined clamping force.
A vibrator 312 is configured of the first elastic body 301, the second elastic body 302, the piezoelectric element 303, the shaft 304 and the lower nut 305 (FIG. 5).
A reference numeral 306 is a slide member and a surface of the lower end thereof comes into contact with the first elastic body 301. The slide member 306 has a structure having a small contact area and an appropriate spring characteristic. A rotor 307 rotates integrally with the slide member 306 because the rotor 307 is fixed to the slide member 306.
In addition, a gear (an output transmitter) 308 is provided on the upper surface of the rotor 307 and the gear 308 rotates integrally with the rotor 307 and transmits the output of the vibration wave driving apparatus to the outside. The upper surface of the rotor 307 has a concave portion and the concave portion engages a convex portion formed in the gear 308.
Furthermore, a flange 310 for mounting the vibration wave driving apparatus is fitted to the gear 308 and the position thereof is fixed in the thrust direction of the shaft 304. A pressurizing spring 309 is provided between the gear 308 and the rotor 307 to apply the pressurizing force to the rotor 307. In addition, in order to prevent wear of the flange 310, a flange cap 310a is pressed into the flange 310.
The laminated piezoelectric element 303 includes electrode groups having two electrodes, respectively. When an AC (alternating current) electric field having a different phase from a power source (not illustrated) is applied to each of the electrode groups, two types of bending vibration orthogonal to each other are excited to the vibrator.
(a) of FIG. 5 is a cross-sectional view of the rod type vibrator 312 and (b) of FIG. 5 illustrates the magnitude of the vibration amplitude in a vibration mode of the vibrator 312.
(b) of FIG. 5 illustrates one of two types of the bending vibration and the other occurs in a direction perpendicular to the paper surface.
Temporal phase difference of 90 degrees is possible between two types of the bending vibration by adjusting the phase of the applied AC electric field. As a result, the vibrator 312 rotates about the axis of the shaft 304 due to the bending vibration.
As a result, ellipsoidal motion is formed on the surface of the first elastic body 301 coming into contact with the slide member 306 and the slide member 306, which is pressurized by the first elastic body 301, is frictionally driven.
Accordingly, the slide member 306, the rotor 307, the gear 308 and the pressurizing spring 309 rotate integrally about the axis of the shaft 304.
The output of the vibration wave driving apparatus is transmitted from the gear 308 to the outside gear via the rotor 307.
In order to restrain whirling about the rotation axis, the inner diameter of the rotor 307 is fitted to the gear 308 and the inner diameter of the gear 308 is also fitted to the flange cap 310a. 
According to the structure described above, a stable contact state is able to be maintained between the rotor 307 and the first elastic body 301 without shifting the rotor 307 from the rotation center or without a moment being acted in which the body of the rotor 307 falls with respect to a sliding surface of the gear 308.
However, it is increasingly preferable that the vibration wave driving apparatus be reduced in size according to downsizing of the camera and the vibration wave driving apparatus of the prior art described above has the following problems in regards to downsizing.
In other words, it is necessary to make the axial length of the fitting portion increased to a certain amount or more in order to avoid a decrease in the output or an increase in loss at the fitting portion between the gear 308 and the flange 310, or to suppress the occurrence of wear of the gear 308 and the flange 310 due to an increase in the contact pressure. This is a major reason hindering the downsizing of the vibration wave driving apparatus of the prior art. Hereinafter, further description on these will be made.
FIG. 4 is a cross-sectional diagram of the fitting portion between the gear 308 and the flange 310.
As illustrated in FIG. 4, when the rotational force is generated in the gear 308, a force is exerted on the gear 308 in the radial direction (direction of arrow A) from an outside gear (not illustrated) which is engaged with the gear 308.
Here, in a case where a clearance δR is excessively large, the gear 308 is inclined (310′) toward the flange 310 and it causes reduction in the output.
Conversely, in a case where the clearance δR is excessively small, sliding loss between the gear 308 and the flange 310 is increased.
In such a circumstance, production in high-precision machining is required to set the clearance δR to a predetermined size. However, this will increase production cost.
Thus, even though the clearance δR has a certain size besides a predetermined one, in order to suppress the inclination of the gear 308 toward the flange 310, the axial length of the fitting portion between the gear 308 and the flange 310 is necessary to be set to a certain length or more.
In addition, in a case where the axial length of the fitting portion between the gear 308 and the flange 310 is short, when the sliding surface receives the force in the radial direction described above, the contact pressure becomes high and the gear 308 or the flange 310 is worn.
Also from such a point of view, the axial length of the fitting portion between the gear 308 and the flange 310 is necessary to be set to a certain length or more.
Thus, a ratio of the axial length of the fitting portion between the gear 308 and the flange 310 is large in the direction of the rotation axis of the vibration wave driving apparatus of the prior art and it becomes a limitation in the downsizing.